
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    * { 
      margin: 0; 
      padding: 0; 
    }
    html, 
    body, 
    #app { 
      margin: 0; 
      padding: 0; 
      height: 100%; 
    }
    #header, #footer {
      height: 50px;
      line-height: 50px;
      text-align: center;
      background: #555;
      color: #fff;
      /* add */
      position:fixed;
      width: 100%;
    }
    #header {
        top:0;
    }
    #footer {
        bottom:0;
    }
    #side { 
      width: 200px; 
      background: #eee;
      /* margin-top: 50px; */
      margin-bottom:50px;
      float: left;
      height: calc(100% - 50px)
    }
    #main {
        margin-top: 50px;
        margin-left: 200px;
        height: calc(100% - 50px);
        border-top: 1px solid transparent;
        box-sizing: border-box;

    }
    #main ul li {
        list-style-type: none;
        float: left;
    }
    #main ul li:nth-of-type(2n)::after{
        content: "|"
    }

    /* css here */
</style>
</head>
<body>
  <div id="app">
    <header id="header">header</header>
    <aside id="side">side</aside>
    <div id="main">
      <ul>
        <li><a href="https://www.bilibili.com/1">Link1</a></li>
        <li><a href="https://www.bilibili.com/1">Link2</a></li>
        <li><a href="https://www.bilibili.com/1">Link3</a></li>
        <br>
        <li><a href="https://www.bilibili.com/1">Link4</a></li>
        <li><a href="https://www.bilibili.com/1">Link5</a></li>
      </ul>
    </div>
    <footer id="footer">footer</footer>
  </div>
  <script>
    // JS here
    document.querySelector('ul').addEventListener('click', function(ev){
        if(ev.target.tagName === 'A') {
            ev.preventDefault();
            alert(ev.target.getAttribute('href'))
        }
    })
</script>
</body>
</html>